A fluid dynamic bearing device supports a shaft member in a non-contact manner with a dynamic pressure effect of fluid formed in a bearing gap. The bearing device of this type has characteristics of high-speed rotation, high rotational accuracy, low noise, and the like, and is recently being suitably used as a bearing device for a motor mounted on various electric equipment such as information equipment, specifically, for a spindle motor of a magnetic disk device including an HDD, or an optical disk device including a CD-ROM, a CD-R/RW, and a DVD-ROM/RAM, and a magneto optical disk device including MD and MO, for a polygon scanner motor of a laser beam printer (LBP), for a color wheel motor of a projector, and for a fan motor.
For example, in a fluid dynamic bearing device incorporated in a spindle motor for a disk drive, such as an HDD, both the radial bearing portion supporting the shaft member in the radial direction and the thrust bearing portion supporting the shaft member in the thrust direction may be formed by fluid dynamic bearings. In a known radial bearing portion in a fluid dynamic bearing device of this type, dynamic pressure grooves as a dynamic pressure generating portion are formed, for example, either in the inner peripheral surface of a bearing sleeve or in the outer peripheral surface of a shaft member opposed thereto, and a radial bearing gap is formed between the two surfaces (see, for example, Patent Document 1).
By the way, in information equipment which incorporates the fluid dynamic bearing device having the above-mentioned structure, for example, a disk drive such as an HDD, further high-speed rotating is required for the purpose of a further increase in speed of reading. In this case, a moment load that acts on a bearing portion supporting a spindle shaft in a freely rotating manner. Thus, in order to deal with an increase in moment load, it is necessary to provide radial bearing portions at multiple points so as to be axially spaced apart from each other, and to increase a span between the radial bearing portions. Further, while there has been adopted conventional structure in which the multiple radial bearing portions are provided on an inner periphery side of one bearing sleeve, downsizing of motor and a reduction in diameters of a spindle shaft and a bearing sleeve involved therewith are required, and hence, in some cases, it is difficult to manufacture a bearing sleeve capable of dealing with an increase in the span between the radial bearing portions.
As means for increasing a span between radial bearing portions and facilitating manufacture of a bearing sleeve, it is considered that multiple bearing sleeves are arranged at multiple points so as to be axially spaced apart from each other (see, for example, Patent Document 2). However, in a fluid dynamic bearing device disclosed in Patent Document 2, it is necessary to arrange a spacer member between the bearing sleeves arranged so as to be axially spaced apart from each other, and hence there arises a problem in that a number of components and an assembly man-hour are increased.
Patent Document 1: JP 2003-239951 A
Patent Document 2: JP 3602707 B